This invention relates to a transducer for effecting conversion between stress wave and electrical energy and to a method of making such a transducer.
The invention has been developed in relation to transducers required for certain forms of apparatus (hereinafter referred to as being of the kind specified) but is capable of application generally to stress wave-electrical tranducers.
Apparatus of the kind specified is intended to provide data relating to an object (including, for example, data as to the existence or position of the object or one or more characteristics of the object) otherwise than by direct vision, and the object data may be ascertained or read either by human perception (as by visual display or by audible or tactile display) or by machine vision.
In a specific form of apparatus of the kind specified the apparatus comprises a combination of
(a) transmitting means for transmitting stress wave energy (hereinafter called the transmitted signal) to a field of view to illuminate the object with such energy,
(b) receiving means for receiving at least part of the energy (herein called the received signal) reflected from the object when in the field of view and effectively forming an image in respect of the illumination of the object by the transmitted signal,
(c) means for imposing predetermined characteristics upon at least part of the signals giving rise to the image in respect of at least one of the parameters pertaining thereto and selected to establish that the image shall contain the required object data,
(d) means for analysing the image derived from the stress wave energy.
The term "image" used herein is to be deemed to include a plurality of signals (readable by human perception or by machine, as appropriate) whether presented serially in time or in spacial juxtaposition or separation and collectively representing the existence, position or attitude of the object, or one or more dimensions thereof, or the configuration or area of the object viewed from any direction.
In a particular form of apparatus of the kind specified the means for imposing predetermined characteristics upon the signals giving rise to the image may differ widely in accordance with the particular data concerning the object which the received signal is required to carry. Thus, if one of the characteristics which the image is required to present is the range or distance of the object or some part thereof from a predetermined point, e.g. the station at which the transmitting means is situated, then the means for imposing the predetermined characteristic may comprise a means for frequency modulating the stress wave transmitted signal in a mode such that the frequency difference between the transmitted signal and the received signal, due to the elapse of transit time between initial radiation of the signal and receipt of the reflection, is representative of the range or distance. In such a case the means for imposing the predetermined characteristic would appropriately impose a frequency sweep on the transmitted signal which may be linear or non-linear but preferably of saw-tooth form.
If the required object data includes information concerning the lateral or angular position of the object, or a part thereof, or the angular width subtended by the object at a predetermined point, e.g. that at which the transmitting means is situated, then the means for imposing a predetermined characteristic may include means to establish that the transmitted signal and/or the received signal is radiated and/or received respectively by way of a beam represented conveniently as a polar diagram with the radius vector centered on a predetermined point and having a maximum value in the central region of said beam and a minimum value at each of two opposite boundaries thereof in a reference plane. The reference plane may be horizontal, vertical, or in some intermediate angular position.
In some cases it may be advantageous to provide transmitting and/or receiving means in which the beam is convergent to a focus or a focal region spaced longitudinally of the transmitting and/or receiving means along an axis extending outwardly into the field of view.
The means for imposing predetermined characteristics on the transmitted and/or received stress wave signals may further include means for scanning the beam angularly through the field of view between boundaries thereof, e.g. in an azimuthal plane or in an elevational plane, or both, or in some intermediate plane.
Transducers for effecting conversion between stress wave energy and electrical energy are required for incorporation in the transmitting means and the receiving means. Possibly a transducer common to both the transmitting means and the receiving means may be used.
The speed or frequency at which such mode of operation is required to be carried out often precludes the use of mounting means for the transducers permitting the transducers to be moved physically to the different angular positions required.
It is known that such beams may be moved angularly or scanned by the use of a transducer which comprises an array of transducer elements connected to the power amplifier or output element of the transmitting means or to the input element of the receiving means through respective channels which include means for imposing a phase difference or time difference between the signals fed to the transducer elements or received therefrom as the case may be, thereby electronically effecting angular deflection of the beam.
The performance of such an array is adversely affected by the need to provide individual transducer elements to form such array and one of the principal objects of the present invention is to avoid or reduce this disadvantage as well as to provide for reduction in the cost of manufacturing such a transducer array.
Further, in cases where it is required to provide a beam either for the transmitting or receiving means of a form which is convergent to a focus or focal region spaced longitudinally from the transducer means along said axis, the need to provide individual tranducer elements imposes considerable complications and contributes to the cost of providing an appropriate array, and imposes considerable limitations on the extent to which the size of the array can be reduced.